With the constantly decreasing feature sizes of integrated-circuit devices, well-behaved devices are becoming increasingly difficult to design. The fabrication is also becoming increasingly difficult and expensive. In addition, the number of electrons within a device is decreasing, with increasing statistical fluctuations in the electrical properties. In the limit, device operation depends on a single electron, and traditional device concepts must change.
Molecular electronics have the potential to augment or even replace conventional devices by molecular electronic elements, can be altered by externally applied voltages, and have the potential to scale from micron-size dimensions to nanometer-scale dimensions with little change in the device concept. The molecular switching elements can be formed by inexpensive solution techniques; see, e.g., C. P. Collier et al, “Electronically Configurable Molecular-Based Logic Gates”, Science, Vol. 285, pp. 391-394 (Jul. 16, 1999) and C. P. Collier et al, “A [2]Catenane-Based Solid State Electronically Reconfigurable Switch”, Science, Vol. 289, pp. 1172-1175 (Aug. 18, 2000). The self-assembled switching elements may be integrated on top of a Si integrated circuit so that they can be driven by conventional Si electronics in the underlying substrate. To address the switching elements, interconnections or wires are used.
Molecular electronic devices, comprising crossed wire switches, hold promise for future electronic and computational devices. Thin single or multiple monomolecular layers can be formed, for example, by Langmuir-Blodgett techniques. A very smooth underlying surface is needed to allow optimal LB film formation. A crossed wire switch may comprise two nanowires, or electrodes, for example, with a molecular switching species between the two electrodes.
Semiconducting electrodes, for example, silicon, are especially useful in such devices. In some cases, the electronic properties of the resulting device can be influenced by the energy band structure of the semiconducting electrode. In addition, using silicon is attractive for compatibility and interfacing with silicon integrated-circuit electronics. Polycrystalline silicon may be especially useful because it can be formed on top of and electrically isolated from the silicon substrate, which can then contain conventional electronic devices.
Therefore, a method of forming polycrystalline silicon electrodes with very smooth surfaces is needed. When polycrystalline silicon is deposited by conventional methods in the polycrystalline form, such as by low pressure chemical vapor deposition (LPCVD) at 0.2 Torr and 625° C. using SiH4, the surface is rough because of the crystalline grains formed during the deposition process; see, e.g., M. Sternheim et al, “Properties of Thermal Oxides Grown on Phosphorus In Situ Doped Polysilicon”, Journal of Electrochemical Society, Vol. 130, No. 8, pp. 1735-1740 (August 1983). If the silicon layer is deposited in the amorphous form (e.g., LPCVD at 0.2 Torr and 525° to 550° C. using SiH4), then the surface is very smooth; see, e.g., E. Ibok et al, “A Characterization of the Effect of Deposition Temperature on Polysilicon Properties”, Journal of Electrochemical Society, Vol. 140, No. 10, pp. 2927-2937 (October 1993); and T. Kamins, Polycrystalline Silicon for Integrated Circuit Applications, p. 148, Kluwer Academic, Boston, Mass. (1988).
What is needed is a process to convert the amorphous form of silicon to the more electrically conductive polycrystalline form, while retaining the smoothness associated with the amorphous form. By “smooth” herein is meant that the rms surface roughness is less than, for example, about 2% of the polycrystalline silicon film thickness.